Transient voltages adversely affect power circuits. Many people mistakenly believe that most problems caused by transient voltages are the result of lightning strikes and black outs. However, the majority of the problems are the result of common mode noise (CMN). CMN occurs between the neutral line and ground line or between a hot line and a ground line. CMN has many causes including, but not limited to, computer power supplies and the starting and stopping of motors. CMN manifests itself as small voltage shifts in the ground line. Further, CMN can be viewed as a higher frequency signal that has been added to the fundamental frequency of the electrical signal. Typically this fundamental frequency is 60 [Hz] while the frequency of CMN can typically exceed 100 [kHz]. Since equipment on a typical electrical network share a common ground, such equipment on the electrical network sees these voltage shifts.
CMN causes logic confusion in microprocessors, which is a common source of malfunctions in these devices. Microprocessors are binary devices that transmit and compute data using a series of ones and zeroes. A microprocessor views 5V with respect to ground as a one, while 0V with respect to ground, a zero. CMN can prevent the voltage seen by the microprocessor from reaching 0V. Thus the microprocessor will mistakenly read a zero as a one causing logic errors. Likewise, CMN can also prevent the voltage from reaching 5V and causing the microprocessor to mistakenly read a one as a zero. Such logic errors can lead to microprocessors misreading data, programs not functioning properly, and even system crashes.
An early solution to filter out CMN was the isolation transformer. However, as power systems became more complex, CMN became more severe and isolation transformers became less economical. New power filtration circuits were developed as a more economical alternative to isolation transformers. An example of a power filtration filter is the Transformer Based Filter (TBF) developed by Smart Power Systems, Inc. Such power filtration circuits used inductors on the neutral line of the circuit and capacitors connected between the neutral and the ground to filter out CMN transient voltage spikes.
Those circuits provide excellent filtration of CMN. However, the presence of neutral-to-ground connections allows current to leak from neutral to ground. Such leakage causes ground fault circuit interrupters (GFCI) to trip. GFCIs are used to protect people from electrical shocks caused by ground faults. GFCIs work by measuring the current entering and leaving the GFCI through the hot and neutral lines. If these currents do not sum to zero then the GFCI opens the circuit. In order to meet Underwriters Laboratories (“UL”) 943 standard a GFCI must trip the circuit before the sum of the currents exceeds 4-6 mA. The National Electric Code (NEC) has added requirements for GFCIs in more locations. The increase in GFCI protected circuits has made it more difficult for individuals to install TBFs to protect sensitive electrical equipment from CMN.
Many power filtration circuits are not compatible with GFCIs because GFCIs view leakage current as a ground fault. If a GFCI views leakage current as a ground fault, the GFCI can trip the circuit. In order to prevent GFCI trips, sometimes purchasers of such power filtration circuits disable the GFCI. Also, isolation transformers disable a GFCI because the GFCI fails to detect what is happening on the secondary side of the transformer. This is unacceptable because the NEC requires GFCI protection in many locations. Further, disabling GFCI protection increases the risk to individuals who are operating devices connected to the power filtration circuits. This issue has led many power filtration circuit manufacturers to specifically state their devices are incompatible with GFCIs.
As such, it would be advantage to have a GFCI compatible system and method for reducing CMN.
Another issue with power filtration circuits is GFCI trips due to un-sequenced relay contact connections. Specifically, when a filter is connected between neutral and ground, if a hot line energizes before a neutral line is connected, some current can pass through the filter to ground causing the GFCI to trip.
As such, it would be advantageous to have a GFCI compatible system and method for activating relay-controlled lines having a filter circuit between neutral and ground.